The “brake-by-wire” operating mode of brake systems, in particular electrohydraulic brake systems, brings about mechanical-hydraulic decoupling of the activation of the brake pedal by the driver. In this context, the driver activates a simulator or a brake pedal sensation simulation device which gives him a pedal sensation which is as pleasant and familiar as possible. However, as a result of this activation brake fluid is not forced directly into the brake circuits as in conventional hydraulic brake systems. Instead, during the activation of the simulator the braking request of the driver is determined and is then included in the determination of a set point braking torque or set point brake pressure. The actual braking is then carried out by means of an active build up of pressure in the brake circuits using a pressure supply apparatus which is actuated by an open-loop and closed-loop control unit. By the hydraulic decoupling of the activation of the brake pedal from the built up pressure it is possible to implement a large number of functionalities, such as ABS, ESP, TCS, slope starting aid, etc. conveniently in such brake systems.
In brake systems of this type, a hydraulic fallback level is usually provided in which the driver of the vehicle can bring the vehicle to a standstill by means of muscle force when activating the brake pedal if the “brake-by-wire” operating mode fails or is disrupted. Whereas in the normal operating mode the hydraulic decoupling, described above, between the brake pedal activation and the buildup of brake pressure is affected by means of a pedal decoupling unit, in the fallback level this decoupling is eliminated, with the result that the driver can force brake fluid directly into the brake circuits.
Modern motor vehicles are generally provided with automatic regulating systems such as ABS, ESP, TCS, which increase the safety of the vehicle occupants to a high degree. They generally intervene when an unstable movement dynamic situation (spinning of the wheels, skidding etc.) is detected and they return the vehicle to a stable movement dynamic state by selective braking and releasing of individual wheels. Whereas in conventional hydraulic brake systems, in which the entire brake pressure has to be applied by the driver by means of muscle force, these regulating systems can only reduce brake pressure selectively and they cannot independently generate any pressure which is higher than the driver's, active, in particular electrohydraulic, brake systems are configured in such a way that pressure can also be independently or actively generated or built up by the actual system.
In regulating processes, pilot pressure is actively built up in a pressure chamber. Pilot pressure denotes here the pressure in the hydraulic pressure chamber in which the pressure piston is slid.
A method for regulating an electrohydraulic brake system, which can be operated, in particular, in the “brake-by-wire” operating mode, is known, for example, from DE 10 2011 076 675 A1. In the aforementioned document, a brake system which has regulating functions such as ABS, TCS, ASR, etc. is regulated, wherein each regulating process is assigned a scaling factor and in each case the smallest of these individual scaling factors is used as the current scaling factor. By taking into account this factor, an intervention of the booster pressure regulator is performed which is more careful compared to the normal braking function in which all the wheel brakes are connected hydraulically to the pressure chamber (with the result that the scaling factor is 1), since the set point rotational speed is scaled down with a scaling factor in accordance with the active regulating functions and therefore does not engage as strongly in the regulating circuit. The magnitude of the scaling factor and therefore also the degree of the reduction in the unscaled pressure regulation production variable depends on which brake regulating or assistance function is active.
A disadvantage with such a regulating strategy is that as a result of the selection of the respective factor the specific braking situation is detected only implicitly and therefore not very precisely. In fact, although in principle account is taken of the fact that wheel-selective regulating interventions which require attenuated intervention by the pressure regulator are performed, account is not taken of how many wheel brakes and which wheel brakes are then actually hydraulically connected to the linear actuator. The scaling factor therefore only describes the current situation, as it were, on average or globally.
Furthermore, in the above-mentioned document it is alternatively or additionally proposed to take into account the number of brakes without implementing this further.
The invention is therefore based on the object of improving a method as described above in such a way that the current braking situation is detected very precisely. Furthermore, a device for carrying out such a method and an electrohydraulic brake system are to be specified.